1. Field of the Invention
The invention relates generally to the field including microdisplay devices and the methods of making them. More specifically, the invention relates to the fabrication of RGB pixels for use in microdisplays.
2. Description of the Related Art
Microdisplays have many military and civilian applications, for example to provide head-mounted displays, hand-held projectors, heads-up displays, and other near-to-eye applications. Microdisplays with high resolution, power efficiency, reliability, and other merits may enable various high-performance portable applications. One category of microdisplays is modulating microdisplays, such as liquid crystal or digital mirror device (DMD) based displays. These are relatively mature in terms of technology, driven by the commercial markets of projection TV and other applications. These modulating microdisplays are blanket illuminated by separate light sources and modulate incident light on a pixel-by-pixel basis, with intrinsically low power efficiency. Due at least in part to this mode of operation, the field-of-view, brightness, and contrast of these modulation-based microdisplays are limited. Another category is emissive microdisplays, which should provide high power efficiency—a critical requirement for portable near-to-eye (NTE) head mounted systems or dismounted mobile systems, especially for field applications. Currently, emissive microdisplay technology is typically based on color-filtered organic light emitting diode (OLED) technology. Although dramatic progress has been made in the OLED field in the last 20 years, the electro-optical performance, power efficiency and lifetime of OLEDs themselves are still inferior to their inorganic counterparts, LEDs. OLED microdisplays suffer not only from a shorter life span but also from nonuniform degradation of luminance for various colors over their lifespan. Furthermore, because of technical difficulties such as conflicting temperatures that may be required for growth of different color organic thin films, and incompatibility with conventional photolithography micro-patterning processes, full-color OLED microdisplays with high resolution based on side-by-side patterned RGB sub-pixels have not been demonstrated. Present day, technically mature approaches utilize filtering of broadband white emitters. For example, a white light-emitting OLED may be grown on a pre-patterned substrate without the necessity for post-deposition patterning, but approximately ⅔ or more of the white light source output must be removed by a filter to obtain the required RGB primary colors. For example, up to 90% of optical power from a white OLED may have to be filtered out in order to obtain a sufficiently saturated red sub-pixel. In such a case, the OLED must be driven up to ten times brighter than the required pixel brightness, which substantially shortens a lifetime of the microdisplay. Degradation is further enhanced by the heat generated when the filters absorb light at wavelengths other than the intended color. Even with its inherent power inefficiency, color filtering OLED microdisplays represent the most widely commercialized emissive microdisplay technology used in military and commercial systems.
In U.S. Pat. No. 6,410,940, the entire contents of which are herein incorporated by reference, a monochrome microdisplay based on semiconductor micro-size emitters is disclosed that can provide high brightness and efficiency, high reliability and long lifetime. The '940 display is not full-color, however. Furthermore, since the fabrication of certain semiconductor emitters may not be compatible with the silicon integrated circuits that provide the current needed to light up, or drive an LED, a/k/a “driver” IC chips, microdisplays formed from such emitters cannot be directly constructed on such IC chips.